Who do you consider to be the "big four" of death metal? You can choose
for a number of reasons: quality, longevity, popularity, influence, etc. which
makes picking really tough. My personal big four are as follows.

Those are just the ones that come to mind when I think of the main death metal bands, I supposed in terms of popularity Cannibal Corpse would be one of them, and Obituary is also one of the main bands as well.